Multiple-input multiple-output (MIMO) wireless communications techniques are used in local area networks (LANs) and 4G cellular networks to enable an access point (AP) to transmit multiple streams of data, concurrently, to a client or user station (STA). MIMO communications offer many advantages over conventional communications techniques, including, for example, high capacity, extended coverage, increased diversity, and/or interference suppression. As a result, multi-user MIMO (MU-MIMO) has emerged as an important feature of next-generation wireless networks. MU-MIMO has the potential to combine the high capacity of MIMO processing with the benefits of space-division multiple access (SDMA).
MU-MIMO communications techniques are described, e.g., in the IEEE 802.11ac specification. In brief, data streams are transmitted from an AP to two or more STAs concurrently. For example, beamforming can be used to transmit a first data stream from the AP to a first STA and a second data stream from the AP to a second STA at substantially the same time. Some APs may include an array of antennas that can be used to transmit and/or receive wireless signals to and from the STAs. More specifically, through beamforming, an AP may utilize multiple antennas to focus energy (e.g., data signals) towards a particular STA in a multi-user environment. The AP relies on channel state information (CSI) to determine the location of the STA and thus the direction in which to focus the signal. However, wireless communications systems typically include mobile devices (e.g., cell phones, tablets, laptop computers, etc.) that often change locations. It is therefore desirable for the AP to adapt its communications parameters based at least in part on the movements of the STAs.